Conventionally, in image reading apparatuses, such as an image scanner and copying machine, a contact image sensor (hereinafter abbreviated as CIS) which is arranged close to an original so as to read it without magnification, has been used as an image sensor which optically reads image information of the original and converts the read information to an electric signal.
FIG. 13A and FIG. 13B depict views for explaining the CIS unit. FIG. 13A depicts a sectional view of the CIS unit, and FIG. 13B depicts a drive circuit of a light source in the CIS unit.
Light emitted from a light source 21 arranged on the end surface of a light guide member 22, is made incident on the light guide member 22, and is guided in the longitudinal direction (perpendicular direction in the figure (main scanning direction)), and linearly irradiates in the main scanning direction of an original 29 placed on an original supporting table 28 such as glass, in a substantially uniform manner. The light source 21 is provided with light emitting elements 21-r, 21-g and 21-b which emit lights having wavelengths of three colors of red, green and blue (hereinafter abbreviated as RGB), respectively. Generally, LEDs of RGB colors are used as these light emitting elements, and the respective light emitting elements are independently driven to be lighted by time-division driving.
In this way, the reflected light from the original 29 illuminated by the light source 21, is converged by a lens array 24, and is formed into an image on a sensor array 25 arranged on a substrate 26. An image signal outputted from the sensor array 25 is outputted to the outside via a connector 27. The light guide member 22, lens array 24, substrate 26 and the like are fixed to predetermined positions by a frame 23.
In FIG. 13B, an FET for driving is connected to each of the light emitting elements 21-r, 21-g and 21-b. Each FET is turned on by making drive signals φLR, φLG and φLB reach a high level, respectively. Thereby, the on-state FET allows current to flow through the light emitting element corresponding to the FET, so that the light emitting element is driven to emit light.
FIG. 14 depicts an external view of the sensor array 25 in the longitudinal direction (main scanning direction), and portions which are common to those in FIG. 13A and FIG. 13B are denoted by the same reference numerals. Note that reference numeral 130 denotes a sensor IC portion constituting the sensor array 25 on an enlarged scale.
Here, the sensor array 25 is not provided with a function to identify color, but merely performs photoelectrical conversion of the incident light quantity. Therefore, here, image signals of an original of respective color components are detected by synchronizing the timing of respective drive signals φLR, φLG, φLB of the RGB light sources and the outputs of sensor array 25 with each other.
The CIS reads an original without magnification, and hence, needs to have a sensor with a length corresponding to the width of the original. To accomplish this, the CIS has a multi-chip constitution in which plural sensor ICs (25-l) to (25-i) are linearly arranged. Here, in each of the sensor ICs, n pixels P(1) to P (n) which perform photoelectric conversion are linearly arranged in the main scanning direction at a predetermined interval x, respectively. For example, in the case of a sensor IC whose resolution is set to 600 dpi, the interval x is set as x=42 μm.
Further, with the increase in the image read speed in recent years, it is required to increase the operating speed of the sensor. In order to cope with this requirement, the operating speed of the sensor array 25 is increased by parallelly taking out the outputs of respective sensor ICs of the sensor array 25 having the multi-chip constitution.
FIG. 15 is a figure explaining an operation timing of the above described CIS.
The above described drive signals φLR, φLG, φLB are successively outputted in synchronous with an external synchronizing signal SP. In this way, a reading operation of RGB color images of one line is completed during a time period of 3 times the SP cycle. Further, the sensor IC 25 is constituted so as to collectively transfer electric charges stored by the respective pixels during the SP period to an analog memory (not shown) provided in the sensor IC 25, and to successively output the transferred electric charges (denoted as OS (R), OS (G) and OS (B)) from the analog memory in succeeding SP cycles.
Hence, since the CIS is a line sensor, it is necessary to move the CIS unit in the subscanning direction relative to the original, in order to acquire two-dimensional information of the original. At this time, for example, when the original is read by the resolution of 600 dpi, assuming that the SP cycle which is a sensor operation cycle period for one color is set to TW (second), the relative moving speed V between the original and the CIS unit is set as V=42 [μm]/3×TW [sec]. While the original is read, the original and the CIS unit are moved relative to each other at the constant speed V. At this time, the irradiation positions on the original in the subscanning direction are different for each of RGB. This causes the RGB read timing to be different for each of RGB. For this reason, when RGB outputs are composed as they are and formed as color image information, the color misalignment is caused in the subscanning direction.
FIG. 16A depicts a view for explaining the color misalignment in the subscanning direction. White squares 1501 to 1503 in FIG. 16A represent reading positions on the original read by a certain pixel at respective points of time T1 to T4 in FIG. 15 shown as a timing chart. For example, the R component information of the original is obtained as reflected light of the light emitted from the light emitting element 21-r which is turned on at the timing of φLR during the period TW. However, the original and the CIS unit are moved relative to each other in the subscanning direction at the speed V, as described above. Thus, when the G component information of the original is read at the subsequent time point T2, the reading position on the original is moved from the position at the time point T1 to the position at the time point T2 in the figure. Each of parallelograms 1504 to 1506 in the figure represents reading areas for respective RGB on the original, when the movement in the subscanning direction is taken into account. Therefore, when the deviation amount of the whole RGB is set to Δy, the reading areas of respective RGB are sequentially deviated by ⅓Δy.
On the other hand, there is proposed a CIS adopting a color decomposition system in which three pixel arrays are provided on the sensor IC used for the CIS, and color filters for RGB are mounted on the respective pixel arrays (Patent document 1).
FIG. 17 depicts an external view of such sensor array in the main scanning direction, and reference numeral 160 denotes an enlarged view of the sensor IC part.
This sensor array is constituted similarly to that shown in FIG. 14 as described above, in that plural sensor ICs which perform photoelectric conversion are linearly arranged in order to realize a long read width. Further, in the respective sensor ICs 35-l to 35-i, as denoted by reference numeral 160 in the figure, pixels which perform photoelectric conversion are arranged at an interval x in the main scanning direction, and three arrays of the pixels are arranged at an interval y in the direction of relative movement between the original and the sensor IC (subscanning direction). Further, three kinds of color filters CF-r, CF-g and CF-b which have transmission wavelength regions corresponding to RGB are arranged on the respective pixel arrays on each sensor IC. This enables the pixels of each pixel array to have the spectral sensitivity corresponding to each color of RBG. This sensor array has a multi-chip constitution in which outputs of the three pixel arrays are arranged to be taken out from one common output line, so as to allow the outputs of respective sensor ICs constituting the sensor array to be parallelly taken out for the purpose of raising the operating speed.
FIG. 18 is a timing chart showing the operation of the color CIS unit shown in FIG. 17.
Unlike in the above described case of FIG. 15, the decomposition of an image signal to each color component is performed by the color filter, and hence, a single white-color Xe tube or LED and the like which has a wide emission spectrum in a visible wavelength region is used as the light source. Thus, one kind of drive signal for controlling lighting of the light source is required, and the light source is kept turned on during the reading operation of the CIS unit. Therefore, in the CIS unit using the sensor IC having the three line constitution provided with such color filters, RGB information for one line of an image can be read during one operation cycle (within the period TW), as shown in FIG. 18.
Even in this case, the two-dimensional image is read, while the original and the CIS unit are moved relative to each other in the subscanning direction. Here, the interval y between the pixel arrays on the sensor IC in the subscanning direction is arranged to be, for example, an integer multiple of the line interval at the resolution of 600 dpi. In such a case, the deviation of reading positions on the original with respect to the line signals of RGB becomes the interval y. Thus, when color information is generated by composing the RGB signals, the positional deviation of each line data is corrected by delaying each line data by the amount corresponding to the interval between the pixel arrays, so that an image without color misalignment is obtained.
FIG. 19A depicts a view for illustrating the deviation of reading positions on the original. In the figure, respective squares represent the reading positions of a pixel line on the original at each of the time points T1 and T2 in FIG. 18 as the timing chart. Further, respective parallelograms 180 to 182 corresponding to each of RGB represent the reading areas on the original for which the relative movement between the original and the CIS unit is taken into account. For example, when the original is read at a resolution of 600 dpi, assuming that the period of SP which is the operating cycle period of the sensor is set to TW (second), the relative movement speed V between the original and the CIS unit is set as V=42 [μm]/TW. Here, when the interval y between the pixel arrays shown in FIG. 17 is set to 42 [μm] which is equal to the interval between the pixel lines, the reading positions for the respective pixel arrays which are separated for each color are also deviated by 42 [μm]. This makes it possible to eliminate the generation of color misalignment by composing image signals of respective colors in consideration of, for example, the delay time (for one line) of one line signal (181) of G with respect to one line signal (180) of R, and the delay time (for two lines) of one line signal (182) of B with respect to one line signal (180) of R.
Patent document 1: Japanese Patent Laid-Open No. 2003-324377
However, when the sensor array shown in FIG. 17 is used, following problems arise. When there are differences in the spectral sensitivity between the sensors and in the RGB ratio of emission spectrum between the light sources, the level differences between respective RGB signals occur, as shown in FIG. 18. Here, when white balance is to be adjusted, it is necessary to amplify color component signals with a low level. Generally, the sensor IC has a low spectral sensitivity in the B component, while the light source has a large emitted light quantity in the R component. As a result, the B component signal level tends to become lower than the R component signal level. For this reason, there is a need to amplify the B component signal. Thus, there is a problem that the noise component contained in the B component signal is also amplified by such signal amplification, and thereby the S/N ratio of the B component signal is lowered as compared with the R component signal. For this reason, the color temperature of the light source needs to be adjusted so as to correspond to the spectral sensitivities of the sensor ICs which constitute the above described CIS unit.
Further, in the above described prior art form, when the CIS unit is used as a reading apparatus of an original, the reading resolution in the subscanning direction may be changed for the purpose of reducing the reading time and of the resolution conversion, or the like. On the other hand, when the reading speed in the subscanning direction is changed, that is, when the reading operation is performed in the state where the resolution in the subscanning direction is different from the resolution in the main scanning direction, the color misalignment occurs in the read image. For example, a case is considered where the reading operation is performed at a resolution of 300 dpi in the subscanning direction by using a CIS unit having pixel arrays of a resolution 600 dpi, as shown in FIG. 17. FIG. 19B is a schematic illustration showing reading positions on the original moved when the reading speed in the subscanning direction is doubled. In the figure, white squares represent the respective reading positions on the original for each of RGB in the points of time T1 and T2 in FIG. 18. Further, parallelograms 183 to 185 represent the reading areas on the original, for which the relative movement between the original and the CIS unit is taken into account. When the resolution in the subscanning direction is reduced by half, and the reading operation is performed at the resolution of 300 dpi, assuming that the SP period which is one operating cycle period is set to TW (second), the speed V′ of relative movement in the subscanning direction between the original and the CIS unit is set as V′=2V=84 [μm]/TW. As described above, when the interval y between the pixel arrays is equal to 42 [μm] which is equal to the interval between the pixel lines of 600 dpi, the deviation between adjacent reading positions for the respective pixel arrays which are decomposed for each color becomes 42 [μm], and hence the distance corresponding to one line delay becomes 84 [μm]. Then, for example, when the line signal (185) of B is delayed by one line, the line signal (185) of B is made coincident with the line signal (183) of R. However, the deviation amount of the line signal (184) of G becomes 0.5 line, for which deviation amount cannot be corrected by the delay processing in the unit of one line.
Also, in the case of FIG. 14, when the resolution in the subscanning direction is reduced, the color misalignment is further increased. FIG. 16B depicts a view illustrating the reading positions when the resolution in the subscanning direction is set to ½ of the resolution in FIG. 16A. Also in this case, the difference between the one line signal (1507) of R and the one line signal (1508) of B does not become an integral multiple of one line. This makes it difficult to perform superimposition of RGB signals by the delay processing in the unit of one line. From this, it is seen that the color misalignment cannot be corrected.
An object of the present invention is to solve the above described problems of the prior art.
A feature of the present invention is to provide a color image sensor which comprises light emitting elements that emit light of respective color components, and an array of sensors that detect image signals of the respective color components, and which is capable of adjusting output signal levels from the respective sensors. A feature of the present invention is also to provide an image reading apparatus using the unit of the sensors and a method for controlling the image reading apparatus.
Further, a feature of the present invention is to provide an image reading apparatus which is capable of preventing color misalignment of image signals obtained by reading an original, even when reading speed in the subscanning direction is changed, and to provide a method for controlling the image reading apparatus.